The biggest difference from other virtual tabletops is, instead of supporting published pen-and-paper systems, my approach is to provide one built-in ruleset, designed specifically for playing in a chat-based medium.

In my view, most "stack-of-books" RPGs are not well-suited for online play, so I took a lot of inspiration from more freeform indie RPGs like Risus and Lady Blackbird. Having a built-in ruleset means it's well-integrated with the interface, and should make it easier to provide match-making within the community.

Some design notes:

The tabletop/dice metaphor is used pretty heavily. A pure-online RPG would probably look and work differently, but I didn't want to depart too much from what people are familiar with.

Since chat-based play is slower than face-to-face, I wanted to limit back-and-forth discussion around mechanics. Skill resolution and attacks are based on one roll, and there are no concept of re-rolls or modifiers that might be added after the fact.

Rolls are d6's with blanks and stars (successes), so that results are purely visual. This way, the chat window isn't filled with math.

The battlemat is mostly for positional reference, and as another way to establish setting. Otherwise, the chat window is the main focus, and a lot of work went into making roleplaying/dialogue easy and enjoyable.

A generic fantasy setting is currently supported, but the freeform trait system makes just about any genre possible.

Rolls. Roll a number of dice equal to your most relevant Base Trait, plus any any Bonus Traits that apply.

Example:/roll Agility 3 + Rogue 1 (e.g. pick a lock, 4 dice)

Stars. There is a 1-in-3 chance a Star (aka success) will come up on each die. 1 Star = Basic tasks, 2 Stars = Hard tasks

Combat. Attack rolls do 1 damage per Star. Weapons and armor mainly serve as "props", unless they are special (e.g. magic). Creative stunts are encouraged, and have effects determined by the GM.

Power Points. Power points are like a combination of Hero points and Mana from other games. They let you improvise powers, stunts, or spells based on your traits, and add one Star to the roll.

Feedback so far seems to indicate that hard core role-players are mainly interested in something that supports their favorite system, so there has been less interest than I had hoped. Fans of light rulesets or freeform play-by-post/chat seem to like it a lot.

Anyway, I would be really interested to hear your opinions on this approach, and any thoughts about online roleplaying in general.